Exemplary embodiments of the present invention relate to a catalyst support system for an exhaust treatment device. More particularly, exemplary embodiments of the present invention relate to independent catalyst support systems.
Catalytic converters are devices that operate to reduce the toxicity of exhaust emissions from internal combustion engines by providing an environment for a chemical reaction involving catalysts in which toxic combustion byproducts (for example, hydrocarbons, in the form of unburned gasoline, carbon monoxide, formed by the combustion of gasoline, and nitrogen oxides, created when heat in an engine forces nitrogen in the air to combine with oxygen) are converted to less-toxic gases. Such devices have utility in a number of fields, including the treatment of exhaust gas streams from automobile, truck, and other internal combustion engines.
A catalytic converter generally comprises one or more catalysts (most often comprising a precious metal component such as platinum deposited on a refractory metal oxide support such as gamma-alumina), a catalyst support (a ceramic or metal carrier material typically comprising a substrate such as cordierite) which carries the catalysts, and a washcoat (to which the catalysts are added before application to the support to make converters more efficient). The catalyst serves to catalyze, for example, the oxidation of carbon monoxide, a poison for any air-breathing animal, to carbon dioxide, the oxidation of hydrocarbons, which produce smog, to carbon dioxide and water, and the reduction of nitrogen oxides, which lead to smog and acid rain, back to nitrogen and oxygen.
Current catalytic converters can utilize multiple catalysts and will typically have multiple independent catalyst “bricks,” that is, catalysts which are carried on a porous support and coated on a substrate disposed within the housing. Some bricks have a plurality of cells providing fluid paths therethrough. The catalyst bricks are generally retained in a converter housing or shell by a compressible mat support material, which is disposed between the exterior of the catalyst bricks and the interior surface of the housing. The compressible support material exerts a retaining force or pressure upon the catalyst bricks. The amount of support desired for each catalyst brick individually may be dissimilar from that of the other catalyst bricks because the catalyst bricks may have inconsistent exterior dimensions and/or compositions with respect to one another, or because the dimensions of the catalytic converter housing may be asymmetrical, so that the areas between the exterior surface of each individual catalyst brick and the interior surface of the converter are inconsistent. Nevertheless, current catalytic converters employ a singular, uniform support blanket, or mat, to secure the multiple catalyst bricks.
The proper mat pressure on a catalyst brick is obtained by taking into consideration the type of mat material or materials, the “gap bulk density” (GBD) for the mat in the annular space it occupies between the catalyst brick and the housing under loading (e.g., compressive force), the mass of the catalyst brick and thus the required support from the mat material (e.g., retention pressures based upon basis weight and/or thermal properties) can vary for each brick, the vibrational loads which the catalyst brick must withstand, the coefficient of friction between the mat and housing and between the mat and catalyst brick, the rate of mat compression during assembly of the exhaust treatment device, and the amount of any over compression of the mat during assembly. Thus, as mentioned above two independent catalyst bricks with a single support mat may not provide the most desired support for each brick since each independent and distinct brick may require different support requirements (e.g., insulative, pressure, erosion, etc.).
Mat support materials are produced in different “basis weights,” that is, mat weight per unit area (e.g., grams/meter2). The mat basis weight selected depends on the brick-to-housing annular space, the tolerance range of the substrate and the shell, and other factors such as the mat thickness required to attain the desired support based upon the mass of the brick, cell size, thermal expansion coefficients and the desired temperature for the outer surface of the housing (e.g., insulation requirements).
The gap bulk density (GBD) typically provided in grams per cubic centimeter (“g/cc”) is one of the most important characteristics considered during the design of an exhaust treatment device because it is an indicator of the pressure on the brick, brick retention force, force on the brick due to mat expansion during vehicle operation or temperature changes, and the rate of mat erosion. The GBD can be obtained for a particular gas treatment device assembly by determining the annular space or “annulus” between the catalyst brick and the inner housing surface, together with the mat's basis weight. The GBD defines the level of mat compression in grams per cubic centimeter (g/cm3).
Variations between the catalyst bricks within a catalytic converter housing of uniform shape, or within a nonuniformly shaped converter housing in some instances, can produce variations in the annulus between the individual catalyst bricks and the inner surface of the housing. When variations such as these cause the annular space to reach a minimum (the “minimum annulus condition”), a condition of maximum gap bulk density is produced. Under this condition, the mat pressure on a catalyst brick can become high enough to cause the brick's substrate to fracture. Since substrates account for about 90% of the total cost of an exhaust treatment device, it is desirable to minimize or eliminate these fractures.
Since excessive mat forces may cause the substrate to fracture, it is desirable to limit the maximum gap bulk density for each catalyst brick individually to ensure proper substrate retention without causing fractures and to limit mat erosion to acceptable levels. Nevertheless, the insulating support system currently used for exhaust treatment devices that utilize multiple catalyst bricks does not specifically account for differences in characteristics such as size, weight, thermal insulation properties, and exhaust gas erosion properties between the mat support and the individual catalyst bricks.
Accordingly, it is desirable to provide a catalyst support system for exhaust treatment devices that utilize multiple catalyst bricks that can account for the dissimilarities in the amount of support desired for each of the catalyst bricks individually.